Selective precipitation of nickel and cobalt

ABSTRACT

A method for precipitating nickel and cobalt from an acid aqueous solution containing at least dissolved nickel, cobalt and manganese, the method including: adding solid caustic calcined magnesium oxide or freshly slurried caustic calcined magnesium oxide to the solution, the magnesium oxide being added in an amount sufficient to precipitate a substantial proportion of the nickel and cobalt in solution and to precipitate a minor proportion of the manganese in solution, maintaining the magnesium oxide in contact with the solution for a period of about 1 hour to about 9 hours to thereby achieve precipitation of a substantial proportion of the nickel and cobalt in solution and precipitation of a minor proportion of the manganese in solution; and separating solids precipitated in step (b) above from the aqueous solution. Preferably, about 80% to 100% of the Ni and Co in solution is precipitated and about 5% to 15% of the Mn in solution is precipitated. The precipitated material separates early from the solution. The precipitate contains low levels of Mn and Mg and can be further treated to recover nickel and cobalt.

[0001] The present invention relates to a method for precipitatingnickel and cobalt from acidic aqueous solutions. The method is suitablefor use in the recovery of nickel and cobalt from ores or concentrates,especially lateritic ores and concentrates obtained from lateritic ores.

[0002] Lateritic ores are commonly treated to recover nickel and cobalttherefrom by pressure leaching with an acid. This results in theextraction of nickel and cobalt from the ore into the aqueous phase. Theleaching step also results in the extraction of other metals in the oreinto the aqueous phase. Typically, manganese, magnesium and iron arealso leached from the ore and a mixed solution containing several metalions is produced.

[0003] Typical nickel-ore processing plants treat the leach solution toproduce a precipitate containing nickel and cobalt and further treat theprecipitate to separately recover nickel and cobalt at a satisfactorypurity. The further treatment of the precipitate may involve a furtherleaching to extract nickel and cobalt, followed by liquid-liquidextraction to separate the nickel and cobalt and recovery stages toseparately recover nickel and cobalt.

[0004] Operating experience with plants that treat nickel ores has shownthat a number of difficulties exist in the treatment of the aqueousphase resulting from the pressure acid leaching of the ore. For example,adding sodium hydroxide or sodium carbonate to the acidic leach solutionresults in a very fine or slimy precipitate being formed which isdifficult to settle and filter. Filter cake washing can also bedifficult due to the small particle size of the precipitate.Precipitation with calcium hydroxide results in the formation of aninsoluble calcium sulphate precipitate, resulting in contamination ofthe nickel/cobalt product. Precipitation of nickel and cobalt as asulphide is selective and gives a precipitate that is readilyfilterable. However, the equipment required to carry out theprecipitation is capital intensive, as is the equipment required toproduce the hydrogen sulphide. The resultant nickel cobalt sulphiderequires pressure leaching to dissolve, which also requires high costequipment. The sulphate that results requires eliminating either asammonium sulphate or sodium sulphate. This requires ammonia or sodiumhydroxide to be used as the neutralising agent, both of which areexpensive.

[0005] Another method of precipitating nickel and cobalt from leachsolutions is to add magnesium oxide to the acidic leach solutions.Precipitation with magnesium oxide should result in the dissolution ofmagnesium to form soluble magnesium sulphate. However, this isfrequently an imperfect operation which results in a nickel/cobaltproduct containing high levels of magnesium.

[0006] All of the above techniques apart from sulphide precipitationalso lack selectivity with respect to manganese precipitation, resultingin a nickel/cobalt precipitate high in manganese.

[0007] An earlier patent recognising some of the above difficulties isAustralian Patent No. 655774 (AU-B-22766/92) in the name of Hoefer. Thispatent discusses the treatment of a liquor from a leaching orbeneficiation circuit for oxidised nickel-containing ore byprecipitating the valuable species and to pass the liquor through athickener/filtration circuit to separate the valuable species from theliquor. The patent states that this is not a satisfactory solution fornickel because the nickel precipitates that can form most readily, suchas nickel hydroxides and sulphides, are gelatinous and difficult tothicken and filter. In particular, the nickel precipitates tend to blindfilters quickly. The patent addresses the problems of thickening andfiltering by adding an inert particulate carrier and a flocculent to theliquor to form flocs. However, this process requires the addition offurther materials to the liquor and does not address the issue ofmanganese precipitation.

[0008] U.S. Pat. No. 2,899,300 in the name of Bailey (assigned to QuebecMetallurgical Industries Ltd) discloses a process for treating nickellateritic ores. The process incudes contacting the ore with sulphuricacid in an amount sufficient to saturate the ore. The acid-saturated oreis dried by baking at a temperature between 100-150° C. and subsequentlycrushed. The crushed ore is then leached with water to obtain a leachsolution containing nickel and cobalt values, as well as iron, manganeseand chromium. The pH of this leach solution is then adjusted to withinthe range of 3.5-4.2 to precipitate ferric iron. After removing theiron-containing precipitate, reactive magnesia (either in powder or milkform) is added to the solution to bring its pH up to about 8.2 tothereby precipitate a nickel-containing concentrate. Practically all ofthe nickel and cobalt is precipitated from solution, along with theremaining iron and about 50% of the manganese. The precipitate is statedto settle rapidly to a dense pulp.

[0009] The example included in this patent treats a lateritic ore havinga low manganese content of 0.26 wt % Mn. The leach liquor has a ratio of(nickel plus cobalt) to manganese in the leach liquor of 11.2. The sameratio in the final precipitate is 17.9, showing that only a relativelysmall concentration of nickel and cobalt relative to manganese, isachieved. In other words, the precipitation is not selective to nickeland cobalt precipitation. Accordingly, the process described in U.S.Pat. No. 2,899,300 would be only suitable for treatment of lateriticores having low manganese contents.

[0010] Furthermore, the precipitated product contains significantquantities of iron (6.2 wt %). This can be deleterious because thepresence of iron in the precipitate can suppress re-leaching of nickeland cobalt from the precipitate.

[0011] U.S. Pat. No. 3,466,144 in the name of Kay (assigned to AmericanMetal Climax, Inc.) describes a hydrometallurgical process forrecovering nickel and cobalt from nickeliferous oxidic ores. In theprocess, the ore is leached with sulphuric acid at elevated temperatureand pressure. The loaded solution is separated from the solid residue.The pH of the loaded solution is increased to about 3.4-4.5 by addinglime or magnesia to precipitate iron, aluminium and silicon whilst thenickel, cobalt and manganese remain in solution. The resultingprecipitate is separated from the solution.

[0012] The loaded solution is then treated by adding magnesia until thepH is at least 8 in order to precipitate the nickel, cobalt andmanganese. The thus-formed hydroxides of nickel, cobalt and manganeseare then separated from the solution (e.g. by vacuum filtration) and thefilter cake is washed with water and sent for further refining.

[0013] U.S. Pat. No. 3,466,144 discloses a two-stage precipitation inwhich iron is first removed from solution, followed by a non-selectiveprecipitation of nickel, cobalt and manganese from solution. Thisresults in a solid precipitate that contains significant quantities ofmanganese.

[0014] U.S. Pat. No. 3,720,749 in the name of Taylor et. al. (alsoassigned to American Metal Climax, Inc.) discloses a process similar tothat described in U.S. Pat. No. 3,466,144 but with the improvement thatthe first stage precipitation to remove impurities such as dissolvediron, aluminium and silicon from the solution is conducted by adjustingthe pH at elevated temperature and pressure. This enables a wider pHrange to be used for the first stage precipitation. The second stageprecipitation to precipitate nickel, cobalt and manganese from solutionmay be conducted by adding a neutralising agent to cause precipitationof hydroxides or by adding H₂S to cause precipitation of sulphides.Example 2 shows the stage 2 precipitation being conducted by adding MgOuntil the pH of the leach solution falls within the range of 5.6 to 8.8.This resulted in precipitation of 88.4% of the nickel, 83.7% of thecobalt, 57.8% of the manganese and 30.6% of the chromium. Clearly, theprocess does not provide for selective precipitation of nickel andcobalt over manganese.

[0015] The present invention provides a method for precipitating nickeland cobalt that overcomes or at least ameliorates one or more of thedisadvantages of the prior art.

[0016] According to the present invention, a method is provided forprecipitating nickel and cobalt from an acid aqueous solution containingat least dissolved nickel, cobalt and manganese, the method including:

[0017] a) adding solid caustic calcined magnesium oxide or freshlyslurried caustic calcined magnesium oxide to the solution, the magnesiumoxide being added in an amount sufficient to precipitate a substantialproportion of the nickel and cobalt in solution and to precipitate aminor proportion of the manganese in solution;

[0018] b) maintaining the magnesium oxide in contact with the solutionfor a period of about 1 hour to about 9 hours to thereby achieveprecipitation of a substantial proportion of the nickel and cobalt insolution and precipitation of a minor proportion of the manganese insolution; and

[0019] c) separating solids precipitated in step (b) above from theaqueous solution.

[0020] Preferably, the method of the present invention further includesthe steps of:

[0021] i) determining the amounts of nickel, cobalt and manganese insolution;

[0022] ii) determining the amount of magnesium oxide required to effectprecipitation of a substantial proportion of the nickel and cobalt insolution and a minor proportion of the manganese in solution; and

[0023] iii) adding the determined amount of magnesium oxide to thesolution.

[0024] Step (ii) above most preferably includes the steps of:

[0025] iia) determining a theoretical amount of magnesium oxide to beadded to the solution to cause the precipitation of a substantialproportion of the nickel and cobalt in solution and a minor proportionof the manganese in solution, said theoretical amount of magnesium oxidebeing determined by stoichiometric requirements to obtain saidprecipitation; and

[0026] iib) adjusting the theoretical amount of magnesium oxidedetermined in step (iia) above by multiplying or dividing thetheoretical amount by an efficiency factor to obtain an addition amountof magnesium oxide, said efficiency factor being determined to accountfor residence time and reactivity of the magnesium oxide.

[0027] The addition amount of magnesium oxide is then added to theaqueous solution. Laboratory and pilot plant testing conducted by thepresent inventors have found that the “efficiency” of the magnesiumoxide is around 70-90%. In other words, about 70-90% of the magnesiumoxide added to the aqueous solution effectively participates in theprecipitation reaction. Thus, the addition amount of magnesium oxide maytypically be calculated by dividing the theoretical amount of magnesiumoxide (determined from stoichiometric requirements) by an efficiencyfactor of 0.7-0.9.

[0028] It is preferred that the substantial proportion of nickel andcobalt in solution that is precipitated comprises from about 80% toabout 100% of the nickel and cobalt in solution, respectively, mostpreferably about 90%. It is preferred that the minor proportion ofmanganese that is precipitated comprises from about 5% to about 15%,most preferably about 8% of the manganese in solution. (All percentagesare given on a weight % basis).

[0029] It is especially preferred that the solution being treated issubstantially free of dissolved iron because dissolved iron may suppressre-leaching of the nickel and cobalt from the precipitate during laterprocessing or refining of the precipitate.

[0030] The precipitant or precipitating agent added to the aqueoussolution comprises solid caustic calcined magnesium oxide or freshlyslurried caustic calcined magnesium oxide. Tests by the presentinventors have discovered that slurried magnesium oxide undergoes an“ageing” phenomenon and becomes less effective as the time fromslurrying increases. Consequently, the most effective precipitant wassolid or freshly slurried caustic calcined magnesium oxide. By “freshlyslurried”, it is meant that the magnesium oxide had been slurried fornot longer than 6 hours prior to mixing with the aqueous solution. Forease of materials handling, it is preferred that the magnesium oxide hasbeen slurried to enable pumping to be used to add the magnesium oxide tothe aqueous solution.

[0031] If solid caustic calcined magnesium oxide is used, it ispreferably in the form of fine particulate matter or a powder.

[0032] To allow the reaction to proceed substantially to completion, areaction time of between one (1) and nine (9) hours is required,preferably from 1 to 6 hours, most preferably from 3 to 5 hours. If theresidence time is less than 1 hour, incomplete dissolution of magnesiumoxide occurs and the solid precipitate recovered is contaminated withmagnesium oxide. If the residence time is greater than about 9 hours,selectivity in precipitation is diminished and the precipitate willcontain higher levels of precipitated impurities.

[0033] The temperature of the precipitation step is preferably fromabout 30° C. to about 90° C., with a temperature of about 50° C. beingespecially suitable.

[0034] It is preferred that the pH of the aqueous solution is adjustedto 4.5 to 6.0 prior to adding the magnesium oxide, although this is notcritical.

[0035] The magnesium oxide added to the aqueous solution must be acaustic calcined magnesium oxide.

[0036] Suitable commercial supplies of caustic magnesia that may be usedin the present invention include CAUSMAG AL4 and CAUSMAG TGM supplied byCausmag International, P.O. Box 438, Young, New South Wales 2594,Australia, and EMAG 75 and EMAG 45 sold by Queensland Magnesia(Marketing) Pty Ltd, PO Box 445, Toowong, Queensland 4066, Australia.Other caustic calcined magnesia may also be suitable for use in thepresent invention.

[0037] The aqueous solution fed to the precipitation process, inaddition to containing nickel, cobalt and manganese ions, may alsoinclude any or all of magnesium, sulphate and chloride ions.

[0038] The aqueous solution recovered from step (c) of the presentinvention may contain unprecipitated nickel and cobalt in solution. Itis preferred that this solution is treated to precipitate the remainingnickel and cobalt, for example, by a non-selective precipitation usingmagnesium or lime as a precipitating agent. The thus-precipitated nickeland cobalt may then be returned to the leaching circuit where the mixedprecipitate is dissolved. A substantial proportion of the manganese mayalso report to the mixed precipitate.

[0039] The method of the present invention results in the formation of anickel-cobalt hydroxide precipitate that has the following properties.

[0040] 1) Low in magnesium;

[0041] 2) Low in manganese;

[0042] 3) Settles and filter readily;

[0043] 4) Is soluble at atmospheric pressure in dilute hydrochloricacid, dilute sulphuric acid, ammonium sulphate solutions, and ammoniacalammonium carbonate solutions.

[0044] The method of the present invention provides for the selectiveprecipitation of nickel and cobalt from acidic leach solutions,especially sulphate, chloride or mixed sulphate-chloride leachsolutions, using magnesium oxide to produce a mixed nickel-cobaltprecipitate which is low in magnesium and manganese and settles andfilters readily. This product in turn is readily releached inhydrochloric acid, sulphuric acid, ammonium sulphate or ammoniacalammonium carbonate solutions. It has surprisingly been found that thesettling and filtration properties of the precipitate are favourable andthe precipitate settles readily, and in fact may be self draining.Vacuum filtration properties are extremely favourable with primaryfiltration rates in excess of 5000 kilograms per square metre per hourbeing measured. This in turn allows the washing of entrained solublesalts to be straight forward.

[0045] The present invention provides a process for the selectiveprecipitation of nickel and cobalt from a leach solution containing atleast nickel, cobalt and manganese. The process allows for selectiveprecipitation of nickel and cobalt over manganese to produce anickel/cobalt containing precipitate having low quantities of manganesetherein. Prior art processes have been unable to achieve selectiveprecipitation of nickel and cobalt over manganese, thus renderingtreatment of lateritic ores or concentrates having manganese thereindifficult or expensive. The precipitate also displays favourablesettling and filtration properties.

[0046] It is particularly preferred that the ratio, by weight, of(Ni+Co)/Mn in the precipitate is at least five (5) times larger than theratio, by weight, of (Ni+Co)/Mn in the solution prior to precipitation.

[0047] A preferred embodiment of the present invention will now bedescribed with reference to the accompanying Figures in which:

[0048]FIG. 1 shows a flowsheet of the precipitation process of thepresent invention; and

[0049]FIG. 2 shows part of a larger flowsheet incorporating theprecipitation process of FIG. 1.

[0050] The flowsheet shown in FIG. 1 may be used in any process whereselective precipitation of cobalt and nickel is required, for example,in the recovery of nickel and cobalt from lateritic ores.

[0051] Referring now to FIG. 1, the feed solution 24 containingdissolved Ni, Co, Mn and possibly other metals such as Mg and Cu is fedto a first reactor 50. Magnesium oxide 51 is also fed to reactor 50. Theresulting mixture of feed solution and magnesium oxide (or magnesiumoxide slurry) passes through two further reactors 52, 53 in order toobtain the desired residence time and plant throughput. After leavingreactor 53, the liquor/precipitate mixture 54 is passed to a thickener55. Underflow from thickener 55 is then passed to a vacuum filter 56 inorder to remove further liquid from the precipitate. Overflow fromhydroxide thickener 55 is sent to a non-selective precipitation step torecover any remaining nickel and cobalt therefrom.

[0052] It will be appreciated that overflow from the hydroxide thickener55 can be treated by a number of methods to recover the residual nickeland cobalt values and eliminate manganese. For example, a non selectiveprecipitation of nickel and cobalt can be carried out using magnesiumoxide or calcium hydroxide as the precipitant, followed by thickeningand recycling of the precipitate to an acid leach. The remainingmanganese containing solution can be further treated with calciumhydroxide and an oxidant if necessary to precipitate the manganese fordisposal. Alternatively, the remaining nickel and cobalt can beprecipitated as sulphides and the manganese containing liquor discarded.

[0053] In the flowsheet shown in FIG. 2, which is part of a largerflowsheet that incorporates the flowsheet of FIG. 1, a loaded orpregnant leach solution 70 is fed to an iron removal process 72 (ifrequired). The solution obtained from iron removal process 72 is thentreated to selectively precipitate nickel and cobalt in accordance withthe present invention. This step is denoted by reference numeral 74 inFIG. 2. It will be appreciated that reference numeral 74 in FIG. 2corresponds to the flowsheet that is upstream of thickener 55 in FIG. 1.Thickener 55 of FIG. 1 corresponds to solid/liquor separation step 76 inFIG. 2. Liquor 78 from solid/liquor separation step 76 (whichcorresponds to the overflow from thickener 55 in FIG. 1) is subjected tonon-selective precipitation 80 by adding magnesia or lime (or any othersuitable precipitating agent) to thereby precipitate any remainingnickel and cobalt values in solution. Solid/liquid separation 82 is usedto recover the mixed precipitate for recycle to the acid leadingcircuit, whilst the solution may be optionally further treated with limeat 84 to precipitate further manganese.

[0054] The present invention will now be described with reference to thefollowing examples.

EXAMPLE 1

[0055] A liquor containing 2.82 g/L nickel, 0.68 g/L cobalt, 2.75 g/Lmanganese and 6.3 g/L magnesium was contacted in an agitated vessel at50° C. for 2 hours with a caustic calcined magnesia known as Causmag AL4at a rate of 3.3 grams of Causmag AL4 per litre of solution.

[0056] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.25 0.015 2.67 7.06 Precipitate (%w/w) 29.7 7.9 3.0 9.9 % precipitated 91.4 97.9 9.0

[0057] It can be seen that over 90% of the nickel and cobalt haveprecipitated, while only 9% of the manganese has precipitated.

[0058] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 1.27:1, in the precipitate it is 12.5:1.

[0059] Based on the above, the efficiency or reactivity of the CausmagAL4 is 72%.

EXAMPLE 2

[0060] A liquor containing 2.69 g/L nickel, 0.66 g/L cobalt, 2.78 g/Lmanganese, and 6.37 g/L magnesium was contacted with a caustic calcinedmagnesia known as EMAG 75 in an arrangement as shown in FIG. 1.

[0061] The addition rate of EMAG 75 was 3.56 g/L, temperature 50° C. andtotal residence time in the reactors was 2 hours.

[0062] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.40 0.058 2.43 6.99 Precipitate (%w/w) 23.9 5.65 2.87 10.2 % precipitated 85.0 91.3 12.6

[0063] While 85% of the nickel and 91.3% of the cobalt haveprecipitated, only 12.6% of the manganese has precipitated.

[0064] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 1.21:1, in the precipitate it is 10.3:1.

[0065] Based on the above, the efficiency or reactivity of the EMAG 75is 64%

EXAMPLE 3

[0066] A liquor containing 4.56 g/L nickel, 1.26 g/L cobalt, 8.76g/Lmanganese and 5.79 g/L magnesium was contacted with a caustic calcinedmagnesia known as EMAG 75 in a continuous pilot plant similar to thatshown in FIG. 1.

[0067] The addition rate of magnesia was 4.63 g/L, temperature 50° C.,and total residence time in the reactors was 3 hours.

[0068] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.636 0.16 8.12 6.55 Precipitate (%w/w) 25.8 7.51 4.14 2.08 % precipitated 86.1 87.3 7.3

[0069] While 86% of the nickel and 87% of the cobalt are precipitated,only 7.3% of the manganese have precipitated.

[0070] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 0.66:1, in the precipitate it is 8.0:1.

[0071] Based on the above, the efficiency or reactivity of the EMAG 75is 84%. Vacuum filtration tests were carried out on slurries produced inthe above manner. Filtration form times of 5 seconds were achieved, withtotal dewatering times of 35 to 45 seconds.

[0072] These correspond to form filtration rates of between 5,000 and7,500kg/hr/m² and total filtration rates of between 700 and 820kg/hr/m².

[0073] Vacuum was applied between 56kpa and 63kpa. Temperature 50° C.Feed slurry 27-31% solids, filter cake 41-44% solids.

EXAMPLE 4

[0074] A liquor containing 4.63 g/L nickel, 0.83 g/L cobalt, 5.60 g/Lmanganese and 6.51 g/L magnesium was contacted with a caustic calcinedmagnesia known as EMAG 75 in a continuous pilot plant similar to FIG. 1.

[0075] The addition rate of magnesia was 4.30 g/L with a total residencetime in the reactors of 292 minutes.

[0076] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.50 0.064 5.07 8.44 Precipitate (%w/w) 34.4 5.81 5.63 1.06 % Precipitated 88.8 91.3 11.4

[0077] While 88.8% of the nickel and 91.3% of the cobalt wereprecipitated based on the mass balance, only 11.4% of the manganese wasprecipitated.

[0078] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 0.97:1, in the precipitate it is 7.14:1.

[0079] Based on the above, the efficiency or reactivity of the EMAG 75is 87%.

[0080] The above discharge liquor containing 0.50 g/L nickel, 0.064 g/Lcobalt, 5.07 g/L manganese and 8.44 magnesium was reacted with calciumhydroxide, added as hydrated lime, at a rate of 11.3 grams of CaO perlitre of solution. This step incorporates non-selective precipitation torecover the remaining nickel and cobalt in solution.

[0081] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.002 0.003 3.77 7.99 Precipitate (%w/w) 7.34 1.38 17.8 1.95 % Precipitated 99.4 95.1 16.6

[0082] This precipitate was recycled to an acidic leach for recovery ofthe nickel and cobalt values.

EXAMPLE 5

[0083] A liquor containing 3.63 g/L nickel, 1.07 g/L cobalt and 7.31 g/Lmanganese was contacted with a caustic calcined magnesia known as Emag75 in a continuous pilot plant similar to FIG. 1.

[0084] The addition rate of magnesia was 4.4 g/L with a total residencetime in the reactors of 184 minutes.

[0085] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.25 0.098 7.06 Precipitate (% w/w)24.2 7.05 3.11 2.03 % Precipitated 93.1 90.8 3.4

[0086] It can be seen that over 90% of the nickel and cobalt haveprecipitated based on liquor analysis, while only 3.4% of the manganesehas precipitated.

[0087] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 0.64:1, in the precipitate it is 10.04:1.

[0088] Based on the above, the efficiency or reactivity of the EMAG 75is 72%.

[0089] The above discharge liquor containing 0.25 g/L nickel, 0.098 g/Lcobalt and 7.06 g/L manganese was reacted with calcium hydroxide, addedas hydrated lime, at a rate of 3.74 grams of CaO per litre of solution.This step incorporates non-selective precipitation to recover theremaining nickel and cobalt in solution.

[0090] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.01 0.006 5.12 Precipitate (% w/w)2.18 0.78 17.7 2.22 % Precipitated 96.0 93.9 27.5

[0091] This precipitate was recycled to an acidic leach for recovery ofthe nickel and cobalt values.

EXAMPLE 6

[0092] A liquor containing 2.80 g/L nickel, 0.67 g/L cobalt, 2.78 g/Lmanganese and 6.31 g/L magnesium was contacted with a caustic calcinedmagnesia known as Emag 75 at a rate of 3.77 grams of Emag 75 per litreof solution, over a period of 2 hours.

[0093] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.29 0.024 2.52 7.11 Precipitate (%w/w) 26.2 6.77 2.64 11.0 % Precipitated 89.6 96.4 9.4

[0094] While 89.6% of the nickel and 96.4% of the cobalt wereprecipitated based on liquor analyses, only 9.4% of the manganese hasprecipitated.

[0095] Whereas the (nickel plus cobalt) to manganese ratio in the feedliquor is 1.24:1, in the precipitate it is 12.48:1.

[0096] Based on the above, the efficiency or reactivity of the Emag 75is 62%.

COMPARATIVE EXAMPLE 1

[0097] A liquor containing 3.27 g/L nickel, 0.814 g/L cobalt, 1.33 g/Lmanganese and 5.54 g/L magnesium was contacted with a slurry of EMAG 75,which had aged for a period in excess of 24 hours.

[0098] The addition rate of EMAG 75 was 10.2 g/L, temperature 50° C. andtotal residue time in the reactors was 5 hours.

[0099] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.008 0.004 0.633 6.46 Precipitate (%w/w) 12.4 3.03 4.77 19.7 % precipitated 99.8 99.5 52.4

[0100] The overdosing of magnesium has resulted in significantly lessselectivity of nickel and cobalt precipitation over manganeseprecipitation.

[0101] Magnesium contamination is high due to overdosing.

[0102] Filtration rates of 400 kg/hr/m² were obtained from this example,which are significantly less than those of example 3.

[0103] In addition, the amount of nickel and cobalt filtered relative tothe total solids is considerably less than example 3.

COMPARATIVE EXAMPLE 2

[0104] A liquor containing 3.24 g/L nickel, 0.806 g/L cobalt, 2.88 g/Lmanganese and 5.25 g/L magnesium was contacted with a slurry of EMAG 75which had aged for a period in excess of 24 hours.

[0105] The addition rate of EMAG 75 was 5.6 g/L, temperature 50° C., andtotal residue time in the reactors was 5 hours.

[0106] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.523 0.147 1.48 7.72 Precipitate (%w/w) 15.3 3.49 6.07 7.41 % precipitated 84 82 49

[0107] The selectivity of nickel and cobalt precipitation over manganeseis significantly less than that of examples 1 to 5. The (nickel pluscobalt) to manganese ratio in the feed liquor is 1.40:1 increasing toonly 3.10:1 in the precipitate.

COMPARATIVE EXAMPLE 3

[0108] A liquor containing 2.69 g/L nickel, 0.66 g/L cobalt and 2.80 g/Lmanganese was contacted with a caustic calcined magnesia known asCausmag AL4 at a rate of 5.3 grams of Causmag AL4 per litre of solution,over a period of 6 hours.

[0109] The final liquor and precipitate assays were: Nickel CobaltManganese Magnesium Liquor (g/L) 0.001 0.002 1.71 Precipitate 21.87 4.9512.21 7.3 (% w/w) % 99.8 99.7 38.9 Precipitated

[0110] Substantially complete nickel and cobalt precipitation has beenachieved. However, the selectivity of the nickel and cobaltprecipitation over manganese is less than examples 1 to 6. The (nickelplus cobalt) to manganese ratio in the feed liquor is 1.19:1 increasingto only 2.19:1 in the precipitate.

[0111] It will be appreciated that the invention described herein issusceptible to variations and modifications other than thosespecifically described. It is to be understood that the inventionencompasses all such variations and modifications that fall within itsspirit and scope.

1. A method for precipitating nickel and cobalt from an acid aqueoussolution containing at least dissolved nickel, cobalt and manganese, themethod including: a) adding solid caustic calcined magnesium oxide orfreshly slurried caustic calcined magnesium oxide to the solution, themagnesium oxide being added in an amount sufficient to precipitate asubstantial proportion of the nickel and cobalt in solution and toprecipitate a minor proportion of the manganese in solution; b)maintaining the magnesium oxide in contact with the solution for aperiod of about 1 hour to about 9 hours to thereby achieve precipitationof a substantial proportion of the nickel and cobalt in solution andprecipitation of a minor proportion of the manganese in solution; and c)separating solids precipitated in step (b) above from the aqueoussolution.
 2. A method as claimed in claim 1 further including the stepsof: i) determining the amounts of nickel, cobalt and manganese insolution; ii) determining the amount of magnesium oxide required toeffect precipitation of a substantial proportion of the nickel andcobalt in solution and a minor proportion of the manganese in solution;and iii) adding the determined amount of magnesium oxide to thesolution.
 3. A method as claimed in claim 2 wherein step (ii) includesthe steps of: iia) determining a theoretical amount of magnesium oxideto be added to the solution to cause the precipitation of a substantialproportion of the nickel and cobalt in solution and a minor proportionof the manganese in solution, said theoretical amount of magnesium oxidebeing determined by stoichiometric requirements to obtain saidprecipitation; and iib) adjusting the theoretical amount of magnesiumoxide determined in step (iia) above by multiplying or dividing thetheoretical amount by an efficiency factor to obtain an addition amountof magnesium oxide, said efficiency factor being determined to accountfor residence time and reactivity of the magnesium oxide.
 4. A method asclaimed in claim 3 wherein the efficiency factor is from 70% to 90% andstep (iib) comprises the step of determining the addition amount bydividing the theoretical amount by 0.7 to 0.9.
 5. A method as claimed inany one of claims 1 to 4 wherein from about 80% to 100% of the nickel insolution is precipitated.
 6. A method as claimed in claim 5 whereinabout 90% of the nickel in solution is precipitated.
 7. A method asclaimed in any one of the preceding claims wherein from about 80% to100% of the cobalt in solution is precipitated.
 8. A method as claimedin claim 7 wherein about 90% of the cobalt in solution is precipitated.9. A method as claimed in any one of the preceding claims wherein fromabout 5% to about 15% of the manganese in solution is precipitated. 10.A method as claimed in claim 9 wherein about 8% of the manganese insolution is precipitated.
 11. A method as claimed in any one of thepreceding claims wherein the solution added to step (a) is substantiallyfree of dissolved iron.
 12. A method as claimed in any one of thepreceding claims wherein the solid magnesium oxide added to the solutionis in the form of fine particulate matter or a powder.
 13. A method asclaimed in any one of claims 1 to 12 wherein a slurry of magnesium oxideis added to the solution, wherein the magnesium oxide has been slurriedfor not longer than 6 hours prior to mixing with the solution.
 14. Amethod as claimed in any one of the preceding claims wherein theresidence time in step (b) is from about 1 hour to about 6 hours.
 15. Amethod as claimed in claim 14 wherein the residence time is from about 3hours to about 5 hours.
 16. A method as claimed in any one of thepreceding claims wherein the weight ratio of (Ni+Co)/Mn in theprecipitate as at least five (5) times larger than the weight ratio of(Ni+Co)Mn in the solution being provided to step (a).
 17. A method asclaimed in any one of the preceding claims wherein the temperature instep (b) is from about 30° C. to about 90° C.
 18. A method as claimed inclaim 17 wherein the temperature in step (b) is about 50° C.
 19. Amethod as claimed in any one of the preceding claims wherein the pH ofthe aqueous solution is adjusted to 4.5 to 6.0 prior to adding themagnesium oxide.
 20. A method as claimed in any one of the precedingclaims wherein the aqueous solution recovered from step (c) is furthertreated to precipitate any remaining nickel and cobalt in solution. 21.A method as claimed in claim 20 wherein the further treatment of theaqueous solution recovered from step (c) comprises a non-selectiveprecipitation.